Refractory products taking such configurations as board and various special shapes have been manufactured heretofore from alumino-silicate ceramic fibers impregnated with colloidal silica for use as high-temperature furnace insulation and the like. According to conventional practice, ceramic fibers comprising silica and from about 45 to about 55% alumina are selected to have a mean diameter of about 2 to 3 microns with random lengths ranging from about 11/2 to about 3 inches. The fibers are slurried with dilute colloidal silica and then vacuum-formed using a mesh mold in order to create a board or other shape. Colloidal silica solutions for use in this process comprise water suspensions and commonly contain from about 15 to about 30% hydrophilic silica particles having a mean diameter ranging between 4 and about 75 millimicrons.
During the vacuum-forming step, part of the colloidal silica sol is withdrawn from the ceramic fiber mat. The fibers are ordinarily slurried initially at a rate of 300 to 400 grams of the dilute silica sol per 100 grams of fiber. Semi-finished products from the vacuum-forming operation are dried in a forced-air oven from 4 to 24 hours at from about 105.degree. to about 500.degree. C.
A serious problem exists with respect to these colloidal silica-loaded, ceramic fiber mats in that the silica particles tend to migrate with the water vehicle during the drying step, leaving crust-like surface layers and a soft, comparatively unimpregnated center. Moreover, the crusty surface layers resist penetration by any subsequently attempted infusion with the silica sol. Accordingly, optimum breaking strengths have not heretofore been obtained.
The incorporation of a surfactant in the silica sol has been proposed in the past in an effort to enhance penetration of secondary infusions; but the addition of such common wetting agents as polyethylene glycol ethers, ammonium lauryl sulfates, and various polyoxyalkylene copolymers have exhibited definitely negative effects, actually inhibiting penetration of the sol in practice.